Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, under certain operating conditions, such as high engine speeds, increased EGR may flow to the compressor, thereby increasing the compressor speed and temperature. Degradation to the compressor may occur if a temperature of gases entering the compressor and/or the speed of the compressor increase above threshold levels. The inventors herein have recognized that an EGR valve disposed in the EGR passage may be closed to reduce EGR flow to the compressor, thereby decreasing a temperature of exhaust gas flowing through and a speed of the compressor. However, the inventors have also recognized that closing the EGR valve may trap hot residual gases within the cylinders and/or first exhaust manifold and may also reduce blowthrough. As a result, engine knock may occur and/or engine power may be reduced.
In one example, the issues described above may be addressed by a system for an engine, comprising: a first exhaust manifold coupled to a first set of exhaust valves and an exhaust passage including a turbocharger turbine; and a second exhaust manifold coupled to a second set of exhaust valves and an intake passage via a first exhaust gas recirculation (EGR passage), the first EGR passage coupled to the intake passage between an intake throttle and a most downstream turbocharger compressor. The engine may additionally include a second EGR passage coupled between the second exhaust manifold and the intake passage upstream of the most downstream turbocharger compressor.
As another example, the issues described above may be addressed by a method, comprising: selectively routing exhaust from a first set of exhaust valves to each of a first EGR passage coupled to an intake passage upstream of a compressor driven by a turbine and a second EGR passage coupled downstream of an outlet of the compressor based on engine operating conditions; and routing exhaust from a second set of exhaust valves to the turbine. In this way, under high engine speed/load conditions and/or when the compressor reaches one or more speed or temperature limits, combusted exhaust gases may be directed to the intake passage, downstream of the compressor, instead of upstream of the compressor. Under other conditions, exhaust gases from combustion may be directed to the intake passage, upstream of the compressor. As a result, compressor degradation may be reduced while increasing engine efficiency and fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.